gui-thread-check.c: kill warnings

Including <string.h> to kill these warnings:

gui-thread-check.c: In function 'simple_method_enter':
gui-thread-check.c:57:2: warning: implicit declaration of function 'strstr' [-Wimplicit-function-declaration]
gui-thread-check.c:57:6: warning: incompatible implicit declaration of built-in function 'strstr' [enabled by default]
gui-thread-check.c:61:3: warning: implicit declaration of function 'strcmp' [-Wimplicit-function-declaration]